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1 3.051/BE.340
Lecture 21
Tissue Engineering
Tissue Engineering: a field that seeks to replace, repair or enhance biological function at the scale of a tissue or organ by manipulating cells via their extra-cellular environment
Objectives:
1. Fulfill a biomechanical role (bone, cartilage) 2. Replace physiological function (liver, nerve) 3. Deliver secretory products (insulin) 4. A combination of the above
3 Main Approaches:
1. Extracorpeal/cell encapsulation (3) 2. In vitro synthesis (1-4) 3. In vivo synthesis (1-4)
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1. Extracorpeal/Cell Encapsulation
Method:
1. Encapsulate cells of interest in semipermeable membrane 2. Implant encapsulated device or connect ex vivo
3. Cells secrete product � therapy 4. Remove/disconnect device when therapy concluded
O2, nutrients
CO2, waste
Cells suspended in gel
T cells, B cells
Semipermeable membrane
Immune system: Ab,
Therapeutic molecules
Advantages: ¾�Natural theraputic response from living cells ¾�Use of nonhost cells—immunoisolation
Issues:
¾�Potential for undesirable immune response from adsorption of complement proteins (similar to blood filtration membranes)
¾�Potential for thrombosis formation (Anti-coagulants used during ex vivo treatment)
¾�Potential for rupture of implanted devices
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Applications Investigated:
¾�Diabetes treatment* ¾�Chronic pain* ¾�Neurodegenerative diseases: ALS (Lou Gehrig’s disease, neuromuscular),
Parkinson’s, Alzheimer’s, Huntington’s disease (progressive brain death) ¾�Dwarfism ¾�Anemia/Hemophelia ¾�Macular degeneration (blindness) ¾�Cancer ¾�Liver Failure* * = clinical trials
Device Examples
Encapsulated Islets (Islet Technology Inc., St Paul, MN) CapCell: implantable membrane-encapsulated islets for glucose regulation
Cells: insulin-producing islets Use: long-term treatment of diabetes Device: alginate-based membrane confines islets Treatment: islets transplanted into patient’s pancreas; patients’ blood flows thru membrane; islets detect glucose level variations & respond through insulin production Status: preclinical trials (islet transplantation in clinical trials)
Islet cells
Alginate-based membranes
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Arbios Systems, Inc. (recently acquired from Circe Biomedical)
HepatAssist System: an extracorporeal, bioartificial liver support system
Cells: primary porcine hepatocytes (pig liver cells) Use: temporary liver function for transplant candidates Device: hollow fiber bioreactor, oxygenator, pump Treatment: plasma circulated through bioreactor and
recombined with blood cells Status: Phase I trials completed
Image removed due to copyright considerations.
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2. In vitro Synthesis
Method:
1. Cells seeded in vitro on scaffold device 2. Cells maintained in culture to expand population &
develop tissue organization (in static culture or bioreactors)3. Device implanted once cell colony is established 4. Device degrades, scaffold replaced by remodeled tissue
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Advantages: ¾�Natural theraputic response from living tissues ¾�Permanent therapy ¾�Allows control and quantification not easily obtained in vivo
Issues:
¾�Cell sources - possibility of rejection - tumorogenicity—cell lines
¾�Full organ restoration challenges (e.g., skin)
Applications Investigated:
¾�Vasculature (resorbable & nonresorbable) ¾�Liver tissue ¾�Nerve tissue ¾�Cartilage* ¾�Cornea* ¾�Bladder* ¾�Skin* ¾�Bone ¾�Ligament ¾�Tendon ¾�Muscle ¾�Heart valve ¾�Heart
Company Updates:
Advanced Tissue Sciences, Inc., La Jolla. CA: filed for bankruptcy under Chapter 11, Oct. ’02
Organogenesis, Canton, MA: filed for bankruptcy under Chapter 11, Sept. ’02; Apligraf artificial skin technology acquired by Novartis
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3. In vivo synthesis
Method:
1. Implant porous scaffold device2. Cellular ingrowth in vivo
3. Scaffold replaced by remodeled tissue
Advantages: ¾�Natural theraputic response from living tissues ¾�Permanent therapy ¾�No cell source problems
Issues:
¾�Uncontrolled biological response to implanted scaffold
Applications Investigated:
¾�Vasculature ¾�Skin* ¾�Bone* ¾�Nerve ¾�Ligament ¾�Cartilage (Knee Meniscus)
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Scaffolds for Tissue Generation
Purpose: replace functions of extracellular matrix (ECM)
ECM functions:
1. cell anchorage 2. cell orientation 3. cell growth 4. mechanical integrity to neo-tissue 5. tissue microenvironment 6. cell differentiation 7. sequester, store & present soluble regulatory proteins 8. blueprint for tissue organization (e.g., biomineralized tissue)
ECM types:
Basal Lamina (basement membrane): directly underlying epithelial cells; contains laminin, collagen, fibronectin, vitronectin
Stromal tissue (interstitial matrix): provides structural integrity; contains matrix-secreting cells (fibroblasts, osteoblasts), collagen, elastin, fibrillin, fibronectin, vitronectin, GAGs, glycoproteins, regulatory proteins
Basal lamina
Stromal tissue
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Resorbable Tissue Engineering Scaffolds:
1. Collagen-matrix e.g., artificial skin drawback: immunogenic
2. Biodegradable polymers: PLA, PGA, PLGA e.g., cartilage
drawback: no adhesion sites (can build in RGD)
3. Hydroxyapatite, Bioglass e.g., bone regeneration
drawback: brittle, low strength
Processing of Tissue Engineering Devices
A. Design Issues
1. Cell density must be sufficiently high to enable tissue formation, deliver therapy
2. Transport of nutrients/oxygen/waste nutrients must reach cells within the scaffold/encapsulation device
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Blood vessel Scaffold/encapsulation device
Co
Cnut
x
Limiting distance from nutrients can be gauged from the Thiele ratio, S
(dimensionless ratio of consumption to supply)
k x2 D = nutrient diffusivity in device (cm2/sec)U Co = nutrient concentration at source (mol/cm3)S DC k = cell nutrient uptake rate constant (mol/sec/cell)
o x = distance from nutrient source (bloodstream) (cm) U = cell density in device (cells/cm3)
S>>1 � cells consume more than can be delivered S<<1 � supply greater than demand S = 1 � supply balances demand; use as limit estimate for device design
A rule of thumb in designing tissue engineering devices: xmax= 500 Pm.
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3. Mechanical support - Critical problem for hard tissue scaffolds
- Influenced by¾�materials choices ¾�processing (orientation of polymers & composites)
4. Tissue organization blueprint
Cell migration guidance chemical & morphology effects (chemo/hapto/durotaxis)
Cell Patterning microcontact printing, microlithography
Spatial Organization of Muliple Cell Types - most organs of more than one cell type - pattern based on different ligands, ligand densities, ligand affinities
B. Scaffold Fabrication
Objective: Continuous, high-surface area scaffolds
1. Fabrics
¾�Woven/nonwoven fibers ¾�Mechanical interlocking � pliable, 3D matrix ¾�Porosity and pore size roughly controlled
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2. Bonded fibers
¾�PGA fibers dipped in PLLA/CH2Cl2 solution ¾�Heat treat fibers at Tg,PGA < T < Tm,PLLA to bond PGA to PGA ¾�Dissolve away PLLA ¾�Improved mechanical properties over fabrics; similar porosity
3. Freeze-dried Foams
¾�Polymer solution immersed in liquid N2 � phase separation ¾�Frozen solvent sublimates leaving porous scaffold ¾�Pore size ~ O of spinodal decomposition � controlled pore structure
4. Salt-leached Foams
¾�polymer solution mixed with uniform salt crystals ¾�Solvent evaporates leaving solid polymer/salt composite
Immerse in H2O to leach out salt¾�¾�Controlled porosities up to 93% (< 2 mm thick)
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2. Encapsulation Membranes
¾�Cast concentrated solution onto substrate (flat or tubular)¾�Substrate immersed into a nonsolvent bath ¾�Coagulation of asymmetric membrane results
Microporous substructure (mechanical rigidity, high flux)
Dense surface layer (bath side) (semi-permeable)
¾�Cells suspended in gel within sealed membrane tube (length ~ 3 cm) ordisk (dia ~ 2-3 cm)
Gel & cells
~3 cm
Semipermeablemembrane
600-1200 Pm
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¾�Membrane characteristics
- Molecular weight cutoff: typically ~30-70 kg/mol (<100 nm dia. pores)
Note: Ab ~150 kg/mol
Molecule/Moiety Size
O2, H2O, salts 2-3 �
Lipids, glucose 10 Å
Serum proteins, endotoxins 100 Å
Viruses 1000 Å
Bacteria 104 Å
White blood cells, platelets 105 Å
- Matrix examples: polysaccharides, alginate/chitosan coacervate, collagen
- Body: PAN-PVC, PP, polycarbonate, cellulose nitrate, acrylic
- Shape & Size: disks vs. tubes
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Disks Tubes
Mass transport Favored diameter restrictions
Susceptibility to high surface area favored clotting increases clot propensity
Cell # 50-100M 5M
Cell # required
Diabetes 109
Clotting factor 107-108
CNS therapies 106-107
D. Current Challenges
1. Micromechanical effectsCell differentiation and growth (especially in load-bearing tissues) can be affected by micromechanical stresses transmitted by the scaffold
2. Cell function deterioration
3. Cross-application to other areas (gene therapy, drug delivery)
4. Multicellular tissues and organs- Complex, multicomponent structures (vascularized tissues)- Regeneration-inducing factors (proteins) only known for blood & bone
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Basic Tissue Cell Types and Functions
Cell type Tissue Function Example
epithelial covers external (ex, skin) & internal (ex, intestine, blood vessel) organ surfaces
endothelial cells
connective houses nerves & blood vessels generation),
cartilage, bone
nerve peripheral nerve
supports other body tissues; fibroblasts (ECM
muscle specialized for contraction; smooth, skeletal, cardiac
generate electrical signals & secrete neurotransmitters
brain cells,
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Cell Regeneration Capability
Category Normal Response to Examples
replic. rate injury
renewing/ High; modest n skin, intenstinal labile via stem cell mucosa, bone
differentiation marrow
Expanding/ Low large n endothelium, stable fibroblasts,
hepatocytes, osteoblasts
Static/ None No replication; heart muscle permanent replaced by scar cells, nerve cells
tissue